In a medium enameled cast-iron casserole, heat the oil. Add the peppers, onion, halved garlic and a pinch of salt. Cover and cook over moderate heat, stirring occasionally, until the vegetables soften, 15 minutes. Add the chipotle powder and cook uncovered, stirring, until the vegetables are lightly browned, 5 minutes. Add the tomatoes and their juices, cover and simmer over low heat for 20 minutes. Scrape the sauce into a food processor and puree. Wipe out the casserole. Strain the sauce into it and season with salt; keep warm.